(1) Field of the Invention
The present invention relates to a waveform equalizer which equalizes the waveform of the signal reproduced from record media such as magnetic tapes, disks and the like, and a digital recording/reproduction apparatus including this waveform equalizer.
(2) Description of the Prior Art
The digital recording/reproduction apparatus such as a digital VTR (Video Tape Recorder) which performs recording and reproduction at high density and at high quality have currently been put to practical use in the field of the business broadcasting, and for the domestic use, the one of the first generation will soon be put on the market. In general, with such a digital recording/reproduction apparatus, a waveform distortion is caused in the signal due to the interference between the codes on the signal transmission path. Therefore, the waveform equalizer removes the distortions in signals read from the recording media caused in the transmission path and subjects the signal to the decoding processing.
Hereinafter, the prior art will be described taking the digital recording/reproduction apparatus which uses a magnetic tape as the recording medium as an example, with reference to FIG. 1 to FIG. 4.
The digital recording/reproduction apparatus shown in FIG. 1 comprises a head 1 for detecting the digital signal recorded in the magnetic tape (not shown); a rotary transformer 2 for transmitting a reproduction signal obtained by the detection with the head 1; a reproduction amplifier 3 for amplifying the reproduction signal transmitted from the rotary transformer 2; a fixed waveform equalizing circuit 4 for equalizing the waveform of the amplified reproduction signal; a clock generating circuit 9 comprising PLL (Phase Locked Loop) for generating a master clock signal; a detection circuit 11 for detecting a digital signal by converting the waveform-equalized reproduction signal into a binary format; a demodulation circuit 12 for demodulating the detected reproduction signal; and an error-correction circuit 13 for correcting errors in the demodulated reproduction signal.
With the conventional digital recording/reproduction apparatus having such a structure, the signal recorded in the magnetic tape is picked up as a reproduction signal by the head 1, and is input to the reproduction amplifier 3 via the rotary transformer 2. This reproduction amplifier 3 amplifies the reproduction signal and outputs the amplified reproduction signal to the fixed waveform equalizing circuit 4. Here, the fixed waveform equalizing circuit 4 comprises a phase compensation circuit 41 and an amplitude compensation circuit 42, and the phase compensation circuit 41 and the amplitude compensation circuit 42 compensate the phase characteristic and the amplitude characteristic, respectively, to equalize the waveform distorted due to the interference between the codes, hence the distortions in the amplitude and the phase are removed to improve the quality of the signal.
Thus waveform-equalized reproduction signal is output to the detection circuit 11 and the clock generation circuit 9. The clock generation circuit 9 generates a master clock signal for synchronizing the motions of the whole reproduction system from the input reproduction signal. The detection circuit 11 converts the reproduction signal into a binary format based on this master clock signal and detects the digital signal. Here, the sampling cycle when the reproduction signal is converted into the binary format is defined as the "bit cycle". Thus detected digital signal is demodulated to the digital signal of before the modulation which is performed at the time of recording in the recording medium by the demodulation circuit 12. The error correction circuit 13 corrects the error in the demodulated digital signal and outputs the corrected digital signal.
The detection method with the above-mentioned detection circuit 11 will now be described. The NRZI modulation has been generally used as the modulation method when a digital signal is recorded on the recording medium. And as the method for detecting a signal recorded by this NRZI modulation method, there can be mentioned a partial response detection method. Among them, the PR (1, 0, -1) detection method which is classified as class 4 by E. R. Kretzmer [E. R. Kretzmer, "Generalization of a Technique for Binary Data", IEEE Transactions on Communications Technology, VOL. COM-14(1), pp. 67-68 (February 1966)] is such that, as shown in FIG. 2 showing the frequency-response curve, the peak of the spectrum at the detection point is in the middle frequency domain, and the spectra on the low frequency domain and the high frequency domain decrease, respectively, hence it has an excellent S/N ratio, as well as it has an advantage in that the detection error is hardly caused because the direct-current component is not contained in the detection point.
The PR (1, 0, -1) detection method will be further described with reference to the signal flow in the PR (1, 0, -1) detection method of FIG. 3 and the motion waveform of FIG. 4. In FIG. 3, the input signal al (the top line in FIG. 4) is predecoded by a predecoding circuit PD constituting a recording system in which the transfer function is expressed by 1/(1-D.sup.2){D: delay operator} and recorded in a recording medium as a recording signal b1 (FIG. 4). This recording signal b1 is differentiated by a magnetic recording/reproduction system A to become a reproduction signal c1 (FIG. 4) at the time of reproduction.
This magnetic recording/reproduction system A corresponds to the head 1.about.the reproduction amplifier 3 shown in FIG. 1, and approximately has a transfer function of 1-D. The reproduction decoder B adds a delay signal d1 (FIG. 2) in which the reproduction signal c1 is delayed by 1 bit period T on the reproduction signal c1 to generate an added signal e1 (FIG. 2). This reproduction decoder B is inserted as a part of the detection circuit 11 of FIG. 1, and has a transfer function of 1+D, and the output thereof is regarded as the detection point.
The transfer function of this reproduction system (from the magnetic recording/reproduction system A to the reproduction decoder B) is expressed as (1-D).times.(1+D)=1-D.sup.2, and the transfer function of the recording/reproduction system combined with the predecoding circuit PD in the recording system having the transfer function (1-D.sup.2).sup.-1 (from the pre-decoding circuit PD to the reproduction decoder B) becomes (1-D.sup.2).sup.-1 .times.(1-D.sup.2)=1, and the input signal a1 appears at the detection point as the added signal e1 (FIG. 4).
Then, the code detection circuit C shown in FIG. 3 (corresponding to the detection circuit 11 shown in FIG. 1) compares the added signal e1 with the threshold V.sub.H on the plus side and the threshold V.sub.L on the minus side to convert the signal into a binary format. That is, when the added signal e1 belongs to the value area not lower than the threshold V.sub.H or not higher than the threshold V.sub.L, the added signal e1 is detected as the theoretical value "1", and when the added signal e1 belongs to the value area between the thresholds V.sub.H and V.sub.L, the added signal e1 is detected as the theoretical value "0", and a comparator output signal f1 (FIG. 4) is generated.
Referring back to FIG. 1, the detection circuit 11 obtains the comparator output signal f1 with the above-mentioned PR (1, 0, -1) detection method, latches it by using a master clock signal g1 (FIG. 4) being input from the clock generation circuit 9 to generate a digital signal h1 (the bottom line in FIG. 4). Thus obtained digital signal h1 becomes the signal which has reproduced the input signal a1 (the top line in FIG. 4). With regard to the details of the PR (1, 0, -1) detection method including other detection methods such as an integrating detection and the like, there is a detailed description in the "Introduction to the magnetic recording" (written by Katsuya Yokoyama, Sogo Denshi Shuppan, 1988 first edition).
According to the waveform equalizer included in the conventional digital recording/reproduction apparatus, since the equalizing characteristic is fixed in accordance with the initial characteristic of the apparatus, even if the equalizing characteristic is set to the optimum value so that the distortion amount in the phase and in the amplitude of the reproduction signal becomes minimum, if the optimum value itself of the equalizing characteristic changes with the change of the characteristic with the lapse of time due to the change of the magnetic tape or the head, or the wear of the head, it becomes difficult to equalize the waveform properly, and the error rate increases.
That is, since the PR (1, 0, -1) detection method does not contain the direct-current component, as described above, it has an advantage in that the distortion in the waveform is not caused with the rotary transformer which does not transfer the direct-current component. Compared with the integrating detection in which the conversion into the digital form is performed by comparing the size to one threshold, however, since conversion into the digital form is performed from the relation to two thresholds V.sub.H and V.sub.L, the noise margin to the threshold decreases.
Therefore, when the equalizing characteristic of the waveform equalizer is fixed, if the characteristic of the reproduction system such as a head changes with the lapse of time, the distortion amount of the added signal e shown in FIG. 4 increases, hence such a problem is caused in the PR (1, 0, -1) detection method having a poor noise margin that a risk of misleading the conversion into a digital form drastically increases.